An equivalent time oscilloscope, which may be referred to as a “sampling oscilloscope,” measures amplitude of a signal under test (SUT) at a sampling instant determined by a repeating trigger signal. Typically, the SUT includes a bit stream having an established length. Generally, the sampling oscilloscope takes high bandwidth samples of the bit stream of the SUT. The sampling oscilloscope displays an “eye diagram,” which is a waveform representation of the bit stream over time, and displays the bit stream itself when the trigger signal is synchronous with a pattern repeat rate of the data stream.
When a sampling oscilloscope samples the SUT, deterministic components of the SUT that are synchronous with the trigger signal are accurately reproduced in the resulting waveform. For example, when the SUT has additive random noise, the statistics of the noise are accurately sampled, but the frequency of the noise is not. Similarly, when the SUT has periodic interference at a certain frequency, which is not a harmonic of the trigger signal, then the statistics of the periodic interference are accurately represented in the resulting waveform, but the frequency of the periodic interference may be changed.
FIG. 1 shows illustrative traces displayed by a conventional equivalent time oscilloscope and an equivalent time oscilloscope according to a representative embodiment, for comparison purposes.
Referring to FIG. 1, trace 110 shows a portion of an NRZ signal having a data rate of 10 Gbps acquired with a conventional sampling oscilloscope. Portions of the sampled signal are not synchronous with the trigger signal, including additive noise and edge jitter, for example. The noise and jitter show up as a fuzzy envelope to the shape of the waveform. Notably, even though the actual frequency of the noise and jitter is well below the data rate of 10 Gbps, the displayed frequency of the noise and jitter appears to be much higher than the data rate. Trace 110 thus illustrates that the frequency content of the portions of the sampled SUT that are asynchronous to the trigger (e.g., the noise and jitter) are not preserved, but are aliased to much higher frequencies in the waveform.
The aliased nature of the asynchronous portions of the sampled SUT presents a problem when applying digital filters to the corresponding waveforms. For example, trace 120 of FIG. 1 depicts a case in which a 30 GHz low-pass filter is applied to the sampled SUT depicted by trace 110. Trace 120 shows that the noise and jitter envelope has been effectively removed from the waveform, and thus does not accurately depict the sampled SUT. The actual frequency of the noise and jitter is well below the 10 Gbps data rate, and therefore should not be affected by a low-pass filter with a 30 GHz cutoff. However, because the noise and jitter were aliased to frequencies well above 30 GHz, they are almost completely removed by the 30 GHz low-pass filter.